A circuit arrangement of this type is already known from the journal "Telcom Report" Vol. 5 (1982), No. 4 (English-language edition), pp. 262 et seq. In the same journal, in the 1981 supplement to the German-language edition, and in particular on pages 7 et seq., 19 et seq. and 49 et seq. other relevant details are discussed. On pages 19 et seq., there are shown and described, among other things, the submatrices of the peripheral line trunk groups (LIUs). Internally, they are connected to the dual central switching matrix ("switching network") and externally, through line terminal units (LTU/DIU) to analog circuits or channels of digital transmission systems. In the above-mentioned English-language edition, concentrators are described. They are connected in their respective master switching center to the submatrices of at least two line trunk groups. Each one of the connection establishment procedures is performed over one or the other of these line trunk groups. Thus, both line trunk groups are initially available for all the connecting procedures. There is a description on page 8 of the above-mentioned German-language edition of the cited journal, there is a description on page 8 of the control channels (designated in the introduction as "data links") that are constantly switched through, in an operational state, by the double switching matrix. In this known case, the central switching matrix and the central processor are provided in duplicate for reasons of reliability. This duplication does not represent any substantial expense in the overall context of a switching center of this kind, because it relates to only a few local components. The peripheral line trunk groups and their appurtenant components, such as the local controller and the submatrix, are present in greater numbers, and consequently on a wider scale, and are therefore not duplicated. In the event of a failure in a line trunk group (for example, in its submatrix or its local controller or its power supply) the proportion of all the connected subscriber lines, concentrator main lines and trunk lines that is affected is relatively so small that the temporary interruption of operation, which is thus so limited, is taken for granted in exchanges of the known type described above.
The control channels mentioned above are the same as the data links referred to in the introduction, which must be provided for the exchange of data which serves both for the processing in the CPU of the dialing information arriving in the line trunk groups and the transmission of program and switching software from the central processor to the local controllers and which thus takes place between the local controllers on the one hand and the central processor on the other, and which are brought for this purpose into operational condition or kept constantly available in the processor. On page 25 of the German-language edition of the above-mentioned journal there is a detailed description of how the local controllers are loaded at the start of the switching operation with programs and fixed data from the central processor through these control channels. Loading procedures of this kind must be performed for each of the local controllers. Performance of the loading procedures is necessary not only when the system is turned on (at the start of the switching operation), but also in stand-by situations, when the switching operation that is being conducted with one of the two parallel switching matrices and one of the two central processors--namely, the central processor assigned to the central switching matrix in question--is continued with the other central switching matrix and the other central processor, instead of with the present central switching matrix and central processor.
These standby situations become necessary, in particular, when breakdowns occur or errors are made--in other words, for the most part, without any advance warning. Errors and breakdowns can be sufficiently serious that immediate standby operation is necessary. Frequently, however, such errors and breakdowns affect only parts of the entire exchange; for example, only a part of the central switching matrix, or only specific functions in it or in the central processor. Otherwise, they may occur sporadically.
In such cases--that is, in most cases where standby operations are necessary--the standby procedures in question can be postponed for a short time. Since the loading procedures described above that are associated with a standby procedure involve large quantities of data and must be performed for each local controller, they are relatively time-consuming. Since these loading procedures must be performed completely before the switching operation itself can be commenced or resumed, standby procedures sometimes result in an undesirably long interruption of operation.
In addition, there are also operating situations in which the program software must be altered, particularly in the local controllers. This means that it must be renewed. There are also operating situations in which operating data that are stored in the local controllers or in the concentrator processors of the concentrators must be changed. These operating data may include subscriber switching data and information about system operating rights that the various subscribers possess, as well as their line situation data and so on. There is thus a considerable number of operating situations in which it is necessary for loading procedures to be performed for the local controllers that are included in the link trunk groups, and, possibly, also for the above-mentioned concentrator processors.